1. Field of the Invention
The invention relates to electrostatic chucks for holding a workpiece and, more specifically, to a method and apparatus for dechucking a workpiece from an electrostatic chuck.
2. Background of the Invention
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor fabrication process system. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrode(s), respectively. The electrostatic force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece.
A typical problem with electrostatic chucks is the difficulty of removing the electric charge from the workpiece and the chuck when it is desired to release the workpiece from the chuck. One conventional solution is to connect both the electrode and the workpiece to ground to drain the charge. Another conventional solution, which purportedly removes the charge more quickly, is to reverse the polarity of DC voltage applied to the electrodes. This technique is described in the context of a chuck having two electrodes (a bipolar chuck) in U.S. Pat. No. 5,117,121 issued May 26, 1992 to Watanabe, et al.
A shortcoming that has been observed with these conventional approaches to removing the electric charge is that they fail to completely remove the charge, so that some electrostatic force remains between the workpiece and the chuck. This residual electrostatic force typically necessitates the use of a mechanical force to separate the workpiece from the chuck. When the workpiece is a semiconductor wafer, the force required for removal sometimes cracks or otherwise damages the wafer. Even when the wafer is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the wafer to pop off the chuck into an unpredictably position from which it is difficult to retrieve by a conventional wafer transport robot.
To more accurately reduce the residual electrostatic attractive force that remains between the workpiece and the chuck, attempts have been made to optimize the dechucking voltage by performing measurements upon the chucked wafer to determine an optimal dechucking voltage and dechucking period. Examples of dechucking arrangements are disclosed in commonly assigned U.S. Pat. No. 5,459,632, issued Oct. 17, 1995, to Birang, et al., and commonly assigned U.S. Pat. No. 5,818,682, issued Oct. 6, 1998, to Loo.
However, when successively processing a plurality of workpieces, these chucking/dechucking methods have not completely eliminated or compensated for chuck dielectric polarization and an incrementally increasing accumulation of residual charge on the chuck surface. The result of such charge accumulation is a progressive increase in the difficulty of dechucking each successive workpiece.
Therefore, there is a need in the art for a method that applies a dechucking signal that compensates for progressive charge accumulation upon the chuck surface when successively processing a plurality of workpieces.